1. Field of the Invention
The present invention relates to air intake systems for carburetor-equipped internal combustion engines and, more particularly, to an air intake system equipped with a mechanism which automatically controls the intake air temperature by adjusting a proportioning valve which mixes cold raw air with preheated raw air for the maintenance of an optimal raw air intake temperature, under changing operating conditions.
2. Description of the Prior Art
The efficiency of carburetion and combustion of an internal combustion engine is highest, when the temperature of the combustion air which is sucked into the engine cylinders, via the filter and carburetor, is maintained within a predetermined temperature range. At higher air temperatures, the engine operates at a reduced volumetric intake efficiency, while lower intake air temperatures tend to cause poor carburetion, sometimes even leading to the dreaded carburetor icing, especially under full load operation. In addition, the intake of very cold raw air will also delay the warmup of the engine during cold start, thereby prolonging operation in a poorly lubricated state. Lastly, a rapid warmup is important for ecological reasons, because the emission of exhaust pollutants is much higher, when the engine runs cold.
The preheating of cold raw intake air can be accomplished rather easily, the numerous solutions proposed for this purpose featuring all a so-called exhaust stove, where raw air is pulled through a duct or channel system whose wall or walls are heated by the hot exhaust gasses. The exhaust stove is therefore preferably associated with the exhaust manifold of the internal combustion engine. A warm air duct connects this exhaust stove with the raw air intake duct, at a junction point located upstream of the air intake filter. Using a suitable proportioning valve, the intake air temperature can thus be adjusted by increasing or decreasing the flows of preheated raw air and of cold raw air which are admitted into the raw air mixing duct which leads to the air intake filter.
A number of prior art air intake systems utilize as a proportioning valve a simple pivotable flapper which, in one end position, closes the warm air duct and opens the cold air duct and, in the other end position, closes the cold air duct and opens the warm air duct. The adjustment drive for the flapper is in many cases a pneumatic membrane actuator which is driven by negative air pressure which is derived from the air intake manifold, downstream of the carburetor, via a vacuum line.
This comparatively simple arrangement is subject to several operational problems which are due, on the one hand, to considerable variations in the level of negative air pressure which is available in the manifold and, on the other hand, to the need for continuously measuring the intake air temperature and for quickly adjusting the flow rates of cold raw air and warm raw air accordingly. The negative air pressure in the intake manifold is highest, when the carburetor throttle is closed, i.e. when the engine is idling, and it is lowest, when the throttle is completely open and the engine operates under full load. In the latter case, the pneumatic membrane actuator is virtually ineffective.
One prior art system of the type described above is disclosed in U.S. Pat. No. 3,726,512, which suggests a flapper-type valve at the junction between the cold air duct and the warm air duct, the pivotable flapper being driven by a pneumatic membrane actuator. In the vacuum line between the engine manifold and the pneumatic actuator is arranged a thermostat-controlled pressure relief valve whose thermostat member is exposed to the temperature of the mixed raw air in the clean air space of the air intake filter. At low ambient temperatures, the relief valve remains closed, so that the negative air pressure of the manifold is transmitted to the pneumatic actuator which responds by pivoting the flapper in the direction of closing the cold air duct and opening the warm air duct. At higher ambient temperatures, the thermostat-controlled relief valve progressively opens an outlet which reduces the negative pressure as it is being transmitted to the flapper actuator, thereby allowing an actuator return spring to pivot the flapper in the direction of opening the cold air duct and closing the warm air duct. Such a system is intended to stabilize at the desired intake temperature. A major shortcoming of this prior art solution is that it is ineffective under full load operation, because of the virtual absence of negative pressure. This condition brings with it the risk of carburetor icing, especially at air temperatures which are just slightly above the freezing point.
Another prior art device is proposed in German Pat. No. 20 09 236, where a similarly arranged flapper is resiliently biased in the direction of closing the cold air duct and opening the warm air duct, moving in the opposite direction, under the flow impact of the air which is being sucked into the engine, so that an increase in total air consumption will increase the flow of cold air and decrease the flow of warm air. The bias on the flapper is obtained with either gravity or a spring. To the flapper is also connected a wax thermostat which, while allowing for a flow-induced movement range of the flapper through a lost motion connection, shifts this range in response to the intake air temperature: A temperature increase will shift the movement range of the flapper in the direction of closing of the warm air duct. This arrangement would seem to eliminate the risk of full load icing, at least theoretically. It has been found, however, that the wax thermostat is responding too slowly for an effective adaptation of the flapper position to changing operating conditions of the engine. Furthermore, it has also been observed that, in its position just behind the air duct junction, the wax thermostat may be exposed to air of a temperature which does not correspond to the temperature of the combined raw air flows, when unmixed currents of colder or warmer air impinge on the thermostat, thereby distorting its response.
Another prior art solution, disclosed in U.S. Pat. No. 3,450,119, features a similar arrangement with a pivotable flapper which is spring-biased towards a position in which the cold air duct is closed and the warm air duct is fully opened. A wax thermostat engages the pivotable flapper with a one-way lost motion connection, thereby acting as an adjustable stop which opposes the flapper movement under the aforementioned spring bias. Also connected to this flapper is a pneumatic actuator whose return spring is opposed to and stronger than the flapper spring. This means that, under full load operation of the engine, the warm air duct is completely closed, regardless of the ambient air temperature, so that the same risk of carburetor icing exists here as in the previously described prior art solutions. Furthermore, the complexity of this arrangement reflects itself in increased production costs, greater risk of malfunction and/or improper adjustment. The numerous connection points between driving and driven parts, converting straight-line motions into angular motions, are subject to friction, with the result that the response of this mechanism is hesitant and lagging, depending on the direction of response.